Flat-panel displays, such as liquid crystal displays, plasma displays, and light-emitting diode displays are commonly used for information presentation. However, the panel sizes of liquid crystal and light-emitting diode displays are limited by the manufacturing processes that are employed. In order to overcome this size limitation, practitioners have employed tiling strategies to make very-large-format, flat-panel displays. Such very-large-format, flat-panel displays employ multiple separate displays aligned edge-to-edge to give the appearance of a single, larger display. Referring to prior-art FIGS. 10-12, a very-large-format, flat-panel display 8 comprises a plurality of individual display tiles 10 aligned edge-to-edge in an array. Within each display tile 10, an active area 12 of display pixel elements 14 is located. The display pixel elements 14 are spatially distributed in a regular array over the display substrate 30 with an inter-pixel gap 22 that depends on the display design and manufacturing process limitations formed between the pixels 14.
However, typical flat-panel displays have perimeter elements such as electrical connectors and edge seals at the perimeter of each display tile 10 to provide connectivity to an external controller 80 (shown in FIG. 10) or to protect the materials comprising the display tile 10. Moreover, separate tiles 10 cannot be perfectly butted together. These perimeter elements and tile spacing form an edge gap 20 and usually take up more space than the inter-pixel gap 22. Hence, when multiple display tiles are butted together in an array, there is an edge seam 24 that forms a visible discontinuity between the pixels at the edges of each display tile 10. Moreover, the edge seam 24 typically has a different color and spacing and can reflect light differently than the pixels 14 or inter-pixel gap 22, making the edge seam 24 visible and objectionable to viewers. FIG. 12 illustrates a cross-section of an OLED device with a substrate 30 and cover 32.
A variety of ways have been proposed in the art to overcome the visually objectionable edge seam. In one approach, the edge seam 24 is made small enough that no interruption in the pixel layout is present. However, this is very difficult and expensive to accomplish. Another approach relies upon employing a fiber-optic faceplate to visually expand the active area 12 in each display tile 10 to match the size of the display tile 10. This approach, however, requires a bulky and expensive fiber-optic array. Yet another approach employs a transparent display tile with one display tile overlapping a second display tile at the edge to locate the pixels in the appropriate position. For example, US Publication No. 2007/0001927 describes a tiled display having overlapping display elements. However, there remain visible artifacts and achieving a completely transparent tile at the edges is difficult. Other art, for example US Patent Publication No. 2003/0011303 by Matthies et al., entitled “PROVIDING OPTICAL ELEMENTS OVER EMISSIVE DISPLAYS” discusses forming a layer over an array of display tiles 10, including an absorptive portion that covers the space between pixels neighboring abutted display tiles 10 to occlude light that is emitted through the edge of the display and made visible to the user. This approach will further increase the contrast of the edge seam 24 with respect to the active area 12 by reducing the luminance within the area of the edge seam 24.
Organic light-emitting diodes (OLEDs) are a promising technology for flat-panel displays and area illumination lamps. The technology relies upon thin-film layers of materials coated upon a substrate. However, as is well known, much of the light output from the light-emissive layer in the OLED is trapped due to the diffuse emission within these devices and internal reflections that occur at material boundaries within the display or between the front display surface and the air in the surrounding environment. Because light is emitted in all directions from the internal light-emitting layers of the OLED, some of the light is emitted at an angle that is nearly perpendicular to the front surface of the display. A portion of this light travels towards the front surface of the display and is emitted through the front surface of the display, forming useful light in a traditional OLED display. A second portion of the light is emitted nearly perpendicularly to the front surface of the device and can be either reflected or absorbed by the back of the display device. Much of this reflected light is then typically able to travel through the front surface of the display to become useful light. However, the light that is emitted in a direction that is not near enough to perpendicular to the front surface of the display is reflected at one of the material boundaries and is unable to escape the device. This light is either eventually absorbed within the device or travels the length of the device and escapes through the edge of the display and is not emitted through the front of the display and therefore does not provide useful light. In general, up to 80% of the light that is emitted by the light-emitting layer can be absorbed or emitted other than through the front of the display and is, therefore, lost.
OLED devices generally can have two formats known as small molecule devices such as disclosed in U.S. Pat. No. 4,769,292 and polymer OLED devices such as disclosed in U.S. Pat. No. 5,247,190. Either type of OLED device can include, in sequence, an anode, an organic EL element, and a cathode. The organic clectro-luminescent (EL) element disposed between the anode and the cathode commonly includes an organic hole-transporting layer (HTL), an emissive layer (EL) and an organic electron-transporting layer (ETL). Holes and electrons recombine and emit light in the EL layer. Tang et at. (Appl. Phys. Lett., 51, 913 (1987), Journal of Applied Physics, 65, 3610 (1989), and U.S. Pat. No. 4,769,292) demonstrated highly efficient OLEDs using such a layer structure. Since then, numerous OLEDs with alternative layer structures, including polymeric materials, have been disclosed and device performance has been improved, Inorganic materials and light emitters, for example quantum dots in a polycrystalline matrix are also known and suffer from the same optical difficulties.
Light is generated in an OLED device when electrons and holes that are injected from the cathode and anode, respectively, flow through the electron-transport layer and the hole-transport layer and recombine in the emissive layer. Many factors determine the efficiency of this light-generating process. For example, the selection of anode and cathode materials can determine how efficiently the electrons and holes are injected into the device; the selection of ETL and HTL can determine how efficiently the electrons and holes are transported in the device, and the selection of EL can determine how efficiently the electrons and holes are recombined and result in the emission of light. It has been found, however, that one of the key factors that limit the efficiency of OLED devices is the inefficiency in extracting the photons generated by the electron-hole recombination out of the OLED devices. Due to the high optical indices of the organic materials used, most of the photons generated by the recombination process are actually trapped in the devices due to total internal reflection. These trapped photons never leave the OLED devices and make no contribution to the light output from these devices.
A typical OLED device uses a glass substrate, a transparent conducting anode such as indium-tin-oxide (ITO), a stack of organic layers, and a reflective cathode layer. Light generated from the device is emitted through the glass substrate. This is commonly referred to as a bottom-emitting device. Alternatively, a device can include a substrate, a reflective anode, a stack of organic layers, and a top transparent cathode layer. Light generated from the device is emitted through the top transparent electrode. This is commonly referred to as a top-emitting device. In these typical devices, the index of the ITO layer, the organic layers, and the glass is about 2.0, 1.7, and 1.5 respectively. It has been estimated that nearly 60% of the generated light is trapped by internal reflection in the ITO/organic EL element, 20% is trapped in the glass substrate, and only about 20% of the generated light is actually emitted from the device and performs useful functions.
Referring to FIG. 13, a prior-art bottom-emitting OLED has a transparent substrate 30, a transparent first electrode 42, one or more layers 44 of organic material, one of which is light-emitting, and a reflective second electrode 46. The transparent first electrode 42, one or more layers 44 of organic material, and a reflective second electrode 46 form an OLED 40 that can be a pixel 14 under the control of active-matrix or passive-matrix elements and a controller (not shown). Light emitted from one of the organic material layers 44 can be emitted directly out of the device, through the substrate 30, as illustrated with light ray 50. Light can also be emitted and internally guided in the substrate 30 and OLED 40, as illustrated with light ray 52. Alternatively, light can be emitted and internally guided in the OLED 40, as illustrated with light ray 54. Light rays emitted toward the reflective second electrode 46 are reflected by the reflective second electrode 46 toward the substrate 30 and then follow one of the light ray paths 50, 52, or 54.
A variety of techniques have been proposed to improve the out-coupling of light from thin-film light emitting devices. For example, diffraction gratings have been proposed to control the attributes of light emission from thin polymer films by inducing Bragg scattering of light that is guided laterally through the emissive layers; see “Modification of Polymer Light Emission by Lateral Microstructure” by Safonov et al., Synthetic Metals 116, 2001, pp. 145-148, and “Bragg Scattering From Periodically Microstructured Light Emitting Diodes” by Lupton et al., Applied Physics Letters, Vol. 77, No. 21, Nov. 20, 2000, pp. 3340-3342. Brightness enhancement films having diffractive properties and surface and volume diffusers are described in International Publication No. WO 02/37568 entitled “BRIGHTNESS AND CONTRAST ENHANCEMENT OF DIRECT VIEW EMISSIVE DISPLAYS” by Chou et al., published May 10, 2002. The use of micro-cavity techniques is also known; for example, see “Sharply Directed Emission in Organic Electroluminescent Diodes With an Optical-Microcavity Structure” by Tsutsui et al, Applied Physics Letters 65, No. 15, Oct. 10, 1994, pp. 1868-1870.
Reflective structures surrounding a light-emitting area or pixel are referenced in U.S. Pat. No. 5,834,893 issued Nov. 10, 1998 to Bulovic et al. and describe the use of angled or slanted reflective walls at the edge of each pixel. Similarly, Forrest et al. describe pixels with slanted walls in U.S. Pat. No. 6,091,195 issued Jul. 18, 2000. These approaches use reflectors located at the edges of the light emitting areas.
Light-scattering techniques are also known. Chou (International Publication No. WO 02/37580) and Liu et al. (US Publication No. 2001/0026124) taught the use of a volume or surface scattering layer to improve light extraction. The light-scattering layer is applied next to the organic layers or on the outside surface of the glass substrate and has an optical index that matches these layers. Light emitted from the OLED device at higher-than-critical angles that would have otherwise been trapped can penetrate into the scattering layer and be scattered out of the device.
U.S. Pat. No. 6,787,796 entitled “ORGANIC ELECTROLUMINESCENT DISPLAY DEVICE AND METHOD OF MANUFACTURING THE SAME” by Do et al. issued Sep. 7, 2004, describes an organic electroluminescent (EL) display device and a method of manufacturing the same. The organic EL device includes a substrate layer, a first electrode layer formed on the substrate layer, an organic layer formed on the first electrode layer, and a second electrode layer formed on the organic layer, wherein a light loss preventing layer having different refractive index areas is formed between layers of the organic EL device having a large difference in refractive index among the respective layers. US Patent Publication No. 2004/0217702 by Garner et al. and U.S. Pat. No. 6,831,407 by Cok similarly disclose the use of microstructures to provide internal refractive index variations or internal or surface physical variations that function to perturb the propagation of internal waveguide modes within an OLED. When employed in a top-emitter embodiment, the use of an index-matched polymer adjacent the encapsulating cover is disclosed.
Light-scattering layers used externally to an OLED device are described in US Publication No. 2005/0018431 entitled “ORGANIC ELECTROLUMINESCENT DEVICES HAVING IMPROVED LIGHT EXTRACTION” by Shiang and U.S. Pat. No. 5,955,837 entitled “ELECTROLUMINESCENT ILLUMINATION SYSTEM WITH AN ACTIVE LAYER OF A MEDIUM HAVING LIGHT-SCATTERING PROPERTIES FOR FLAT-PANEL DISPLAY DEVICES” by Horikx, et al. These disclosures describe and define in detail properties of scattering layers located on a substrate. Likewise, U.S. Pat. No. 6,777,871 entitled “ORGANIC ELECTROLUMINESCENT DEVICES WITH ENHANCED LIGHT EXTRACTION” by Duggal et al. describes the use of an output coupler comprising a composite layer having specific refractive indices and scattering properties.
However, none of the light-extraction techniques disclosed serve to reduce the size or visibility of edge seams in a tiled display system. There is a need therefore for an improved emissive device structure in a tiled display system.